They can have even more energy if initiated by a "lateral blast" of the volcano. Images of the snapped, stripped, and stacked logs after the Mount St. Helens eruption vividly illustrate the power of a blast-enhanced pyroclastic flow.
Pyroclastic density currents are most hazardous on, and immediately surrounding, the volcanic edifice. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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Interactives Any interactives on this page can only be played while you are visiting our website. Related Resources. View Collection. The Hazards of Pyroclastic Flows. View Article. Plate Tectonics and Volcanic Activity. Mount Nyiragongo. View Photograph. Educational Resources in Your Inbox. Pyroclastic flows generally follow valleys or other low-lying areas and, depending on the volume of rock debris carried by the flow, they can deposit layers of loose rock fragments to depths ranging from less than one meter to more than m up to about ft.
Skip to main content. Search Search. Volcano Hazards Program. Pyroclastic flows move fast and destroy everything in their path. Pyroclastic flows form in different ways: Collapse of eruption column: during a highly explosive eruption, the column ejected upwards into the atmosphere cools and can become too cool and dense to maintain upward momentum. The lowermost part of the eruption column is called the gas thrust region.
Here, the density of the eruption column is greater than the density of the surrounding air. The column continues to rise, however, because of the thrust provided by the release and rapid expansion of volcanic gas. Occasionally, the gas thrust region may become so chock-full of debris that its high density cannot be supported by the thrust of expanding gases. The column thus collapses downward under gravity as a mass of vesiculating pumice that advances rapidly down the flanks of the volcano.
This is partly attributed to their lower densities, but also to their greater store of kinetic energy generated by vertical drops up to several kilometers above the volcano's summit. The further it falls, the greater its kinetic energy, and the further and faster it will travel horizontally. Pumice flows have a tripartite division. The main body hugs the ground surface and is dominated by pumice fragments in an ash matrix.
An additional component of a pumice flow is the ground surge. These are forward-springing jets of incandescent ash that occur in the advancing head of the flow. They advance with a rolling and rapidly puffing movement which is thought to be caused by the ingestion of air in the front of the flow. Air ingestion produces strong fluidation in flow front, and explosive heating of the air causes some of the material to be hurled forward as a low-density, turbulent surge.
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